With barebottoms you see every little poo, so you're far more likely to clean the tank more regularly. Also, most people who use gravel seem to follow the lb/gal rule which doesn't apply to most tanks or situations. In the case of goldfish tanks, an amazing amount of mulm can build up within the gravel if one is inattentive for the briefest period of time.
I prefer a skim of turtle pebbles on the bottom of my goldfish tanks just for asthetic reasons and to entertain the fish who like to mouth the pebbles. I provide for extra biofilter surface within my filters.